The present invention relates to a device for eliminating stray pulses occurring at the output of a digital-to-analogue converter and which may be caused by for example shortcomings, such as insufficient synchronism, of input data bits. A possible stray pulse in the digital-to-analogue converter is prevented by means of the eliminating device from reaching a following circuit during a time period in which the pulse is predicted to occur.
The need for an eliminating device of the kind set forth in the opening paragraph is well known to people working with random scan displays and similar devices where the signals for the deflection amplifiers are generated digitally and converted to vector signals in a digital-to-analogue converter (D/A converter).
The stray pulse may be a current or voltage pulse occurring at the output of the D/A converter and may be caused, for example, by the input data bits not being perfectly synchronous (internally and/or externally). The stray pulse is often measured in terms of a charge. Since the stray pulses are only predictable in time and not in charge they cannot in general be filtered out by using linear filters.
The method commonly used involves the blocking of the signal supplied by the D/A converter during the time period in which a stray pulse is predicted to occur so that the signal containing stray pulses is substituted by a high frequency pulse train with a fairly constant charge which is easily filtered out, whereby only a direct current component remains. It is possible to filter out the high frequency pulse train because the clock frequency of the D/A converter is much higher than the upper limiting frequency of systems, such as deflection systems coupled through the output of the eliminating device. It is essential that the signal converted in the D/A converter do not pass through any devices which can only pass low frequency signals before it reaches the eliminating device, because such a device prolongs the duration of the stray pulses.
A previously known device for eliminating stray pulses comprises a capacitor, an analogue switch and a buffer amplifier. When the occurrence of a stray pulse is predicted the switch disconnects the connection between the D/A converter and the capacitor. The capacitor stores the prevailing signal across the capacitor at the moment of disconnection and when the switch is reclosed the capacitor is rapidly charged to the new signal value given by the D/A converter.
The known device has several disadvantages. Most of the D/A converters for high frequencies have current output. The known device therefore has to be completed by an amplifier at the input converting the current output of the D/A converter to a voltage output suitable for the stray pulse eliminating device according to the above. Consequently, the amplifier, having a limited bandwidth introduces an undesired prolongation of the duration of the stray pulses.